IP Addressing and Subnet Masking

IP addresses are 32 bits long and are represented as a quad octet set. Each octet represents 1 byte (8 bits) and has a range of 0-255 for 256 values ( 2 8 = 256 ). IP networks are divided into classes, with each class having a network ID and host range. Each class uses bits from the first octet as part of the network ID and depending on the network class can use up to two other octets for the network ID with the remaining octets reserved for hosts on the network. For example, class A networks use the first octet as the network range and the other three octets are for hosts on the networks and class C networks use the first three octets from the left as the network ID, leaving only one octet available for hosts. The Network classes are divided into classes as follows.

IP Address Classes

Class

First Octet's Range Decimal

First Octet's Binary Range

NetworkHost

Default Subnet Mask

A

0-126

00000000 - 01111110

Network.Host.Host.HostExample10.128.22.100

255.0.0.0

B

128-191

10000000 - 10111111

Network.Network.Host.HostExample172.12.15.65

255.255.0.0

C

192-223

11000000 - 11011111

Network.Network.Network.HostExample209.38.1.2

255.255.255.0

DE

224-239
240-255

11100000 - 11111111

D is for multicastE is experimental

N/A

Some IP addresses are reserved for special use and are not to be used as networks.

Reserved IP Addresses

Address

Use

127.0.0.1

Used for loopback

0.0.0.0

Used as default route on Cisco Equipment

255.255.255.255

Broadcast to all nodes on network

IP Network addressing

A network address uniquely identifies each network.

Every machine on the network shares the same network portion of it's IP address.

The node portion of the IP address uniquely identifies the node on its network, can also be called host address.

Identifying the Parts of IP Network Addresses

This table has one example with the other fields left blank for your practice.

Address

Class

Network Portion

Host Portion

Default Subnet

63.125.22.14

A

63.

125.22.14

255.0.0.0

199.2.2.10

189.88.25.1

16.12.1.84

145.1.40.2

221.220.21.20

Process For Subnet masking

How many subnets?
2(masked bits) - 2 = Subnets

How many valid hosts per subnet?
2(unmasked bits) - 2 = Hosts

What are the valid subnets?
256-(subnet base)=Base number

What are the valid hosts in the subnets?
All numbers between subnets minus the all 1s (.255) and all 0s (.0) host addresses.